Metacrangon Zarenkov, 1965

Komai, Tomoyuki, 2012, 3468, Zootaxa 3468, pp. 1-77 : 6-9

Genus Metacrangon Zarenkov, 1965

Metacrangon Zarenkov, 1965: 1764 ; Butler 1980: 113; Holthuis 1993: 291.

Type species. Crangon variabilis Rathbun, 1902 . Original designation. Gender: feminine.

Diagnosis. Body integument firm. Rostrum usually short, without lateral teeth. Carapace with 1 or 2 teeth on dorsal midline; gastric region depressed below, posteriorly defined by low transverse ridge, usually with submedian tooth on either side of midline (submedian teeth reduced or absent in M. clevai sp. nov. and M. trigonorostris ); hepatic tooth accompanied by distinct hepatic groove; antennal, branchiostegal, and pterygostomial teeth present, branchiostegal tooth moderately strong to strong, produced anteriorly or anterolaterally; longitudinal suture extending from base of antennal tooth to about midlength of carapace. Abdomen slightly to moderately sculptured; middorsal carina present or absent; sixth somite with distinct dorsolateral carina extending to posterior margin, making pleuron apparently flared laterally. Telson dorsally sulcate, usually with 3 pairs of dorsolateral spines. Cornea well developed, faceted. First pereopod unarmed on ventral margin of merus; exopod absent. Second pereopod normally developed, not much shorter than other pereopods, chelate; cutting edges of fingers pectinated with spinules. Fourth and fifth pereopods unequal, latter distinctly smaller than former; dactyli of fourth and fifth pereopods flattened dorsoventrally, usually subspatulate or spatulate. No arthrobranch above base of third maxilliped. Pleopods devoid of appendices internae on endopods. Appendix masculina on male second pleopod arising proximal to midlength of mesial margin, much shorter than endopod, bearing spiniform setae. Uropodal exopod with small posterolateral tooth; diaeresis absent.

General description. Body robustly built; integument firm; surface with short setae in carapace, though density variable, almost glabrous in abdomen.

Rostrum variable in shape, but never laterally compressed, always lacking lateral teeth; ventral surface carinate or rounded. Carapace somewhat depressed dorsoventrally; dorsal midline bluntly to sharply carinate, armed with 1 or 2 teeth, anterior tooth, if present, rostral, postrostral or epigastric in position, posterior tooth arising posterior to midlength of carapace (cardiac in position); gastric region shallowly depressed below, posteriorly defined by low transverse ridge, usually having submedian teeth; hepatic tooth always present, accompanied by distinct hepatic groove; antennal tooth acute or subacute; branchiostegal tooth strong, produced anteriorly or anterolaterally, distinctly overreaching antennal tooth; pterygostomial angle concealed by branchiostegal tooth in lateral view, with tiny tooth; postorbital ridges usually low, blunt, extending beyond midlength of carapace, diverging posteriorly, often confluent with rostral lateral margin; branchiostegal ridge distinct; orbital margin evenly concave, usually with cleft at point crossing with lateral rostral ridge; longitudinal suture extending from base of antennal tooth to midlength of carapace; anteroventral part of carapace inferior to branchiostegal tooth with numerous long setae.

Thoracic sternum widened posteriorly, generally flat in males and non-spawning females, deeply concave in spawning molt of females; median keel on fifth to eighth sternites forming sharp tooth on respective somite in males and non-spawning females, strongly reduced to faint median ridges or completely absent in spawning molt of females. First to fifth abdominal sternites each with sharp median tooth decreasing in size posteriorly in males and non-spawning females, these teeth reduced in spawning molt of females.

Abdomen barely to somewhat sculptured, widened in spawning molt. Fourth somite with rounded posteroventral margin. Fifth somite with blunt to acute posteroventral angle. Sixth somite with distinct dorsolateral carinae; pleuron somewhat flared laterally, bearing posteroventral tooth; posterodorsal margin slightly to strongly produced. Telson gradually tapering posteriorly to acute or subacute tooth; dorsal surface grooved medially, dorsolateral ridges distinct, with 2 pairs of tiny spines (only in M. hikurangi are dorsolateral spines absent); posteromedian tooth usually flanked by 1 pair of minute spines and 2 or 3 pairs of stiff plumose setae.

Cornea well developed, faceted, usually darkly pigmented; eyestalk with dorsal tubercle. Antennular peduncle reaching or overreaching midlength of antennal scale, with numerous setae on mesial margin; first segment longer than distal 2 segments combined, with small, acute tooth on ventromesial ridge medially; dorsal surface of first segment concave to accommodate eye; stylocerite with somewhat elevated lateral margin; third segment shorter than second segment; lateral flagellum sexually dimorphic, longer than peduncle in males, shorter than that in females, articles more numerous in males than in females; mesial flagellum shorter than lateral flagellum in males, longer than it in females, with lateral and mesial rows of setae, each article further with covering of minute setae. Antenna with basicerite bearing ventrodistal tooth; flagellum with lateral and mesial rows of stiff setae.

Mandible ( Fig. 34A, B) 4-toothed terminally. Maxillule ( Fig. 34C) with suboval, spatulate coxal endite; basial endite curved mesially, with double row of long spines on truncate mesial margin; endopod slightly curved, bilobed distally, internal lobe with 1 apical spiniform seta ( Fig. 34D). Maxilla ( Fig. 34E) with poorly developed endites; endopod weakly curved mesially, distal portion slender; scaphognathite moderately broad, posterior lobe elongate subovate, with elongate setae on posterior margin. First maxilliped ( Fig. 34F) with poorly developed endites; endopod slender, flattened; exopod with narrow caridean lobe fringed with plumose setae, flagellum well developed; epipod large, faintly bilobed, proximal lobe terminating acutely or subacutely. Second maxilliped ( Fig. 34G) 6-segmented with basis and ischium fused; dactylus subsemicircular, with cluster of short stiff setae and longer setae terminally, mesial margin with long spines proximally; propodus longest, with long spiniform setae and stiff setae on mesial margin; exopod tapering distally, flagellum well developed; epipod subtriangular in shape. Third maxilliped moderately long; ultimate and penultimate segments flattened dorsoventrally, margins fringed with dense setae (lateral setae longer than mesial setae), dorsal surfaces also setose; ultimate segment terminating in minute corneous spinule; antepenultimate segment somewhat flattened dorsoventrally, with cluster of spinules on ventral surface subdistally; exopod not reaching distal margin of antepenultimate segment, with well-developed flagellum.

First pereopod stout, slightly overreaching antennal scale; distomesial tooth (thumb) of palm always fixed; carpus at least with ventrolateral distal tooth; merus with dorsodistal tooth, ventromesial margin carinate or crested, unarmed; no exopod. Second pereopod slender, chelate, overreaching midlength of antennal scale, carried flexed normally; dactylus about 0.3 length of palm; cutting edges of fingers pectinated with minute spinules ( Fig. 35F); carpus longest; merus and ischium subequal in length; coxal process prominent, curved, terminating in acute tip (e.g., Fig. 9E). Third pereopod slender, overreaching antennal scale at least by length of dactylus; dactylus terminating in slender hood bearing sparse minute setae ( Fig. 35H); carpus longer than dactylus-propodus combined; merus subequal in length to ischium; ischium slightly curved, with row of setae on ventral margin. Fourth and fifth pereopods ambulatory legs, moderately slender to stout, slightly to distinctly unequal in length, slightly dissimilar in general structure. Fourth pereopod with subspatulate or spatulate dactylus; tip of dactylus with small unguis flanked by terminal prolongations of corpus ( Fig. 35J); outer surface of dactylus slightly concave at least in dorsal half; propodus-dactylus articulation with rotation of 50–80°; propodus compressed laterally, its outer surface flat to slightly concave, dorsal and ventral margins with rows of stiff setae; carpus shorter than propodus, with numerous long setae on dorsal margin; merus and ischium with row of long setae on dorsal and ventral margins (setae on dorsal margin longer than those on ventral margin); merus longer than ischium. Fifth pereopod shorter than fourth pereopod in total length, but distal three segments slightly shorter to longer than that of fourth pereopod, and less setose than fourth; dactylus narrower than that of fourth pereopod, subconical to spatulate; propodus-dactylus articulation showing weak degree of rotation.

Branchial formula summarized in Table 2.

Pleopods devoid of appendices internae. Appendix masculina on male second pleopod much shorter than endopod, bearing several simple stiff setae on margins and dorsal surface ( Fig. 36C); endopod bearing plumose setae on margins. Uropod with protopod bilobed at posterolateral angle; exopod with small posterolateral tooth, lacking diaeresis; endopod elongate ovate, slightly longer than exopod.

Egg size large (1.3–2.5 mm in longer axis).

Sexual dimorphism. Like in other crangonids, species of Metacrangon exhibit sexual dimorphism in various structures: body less stout in males than in females, teeth and carinae on carapace and abdomen usually more pronounced in males than in females; cornea proportionately larger in males than in females; outer antennular flagellum longer and stouter than inner flagellum in males, shorter than and subequal in width to inner flagellum in females, articles composing outer flagellum more numerous in males than in females. First, fourth and fifth pereopods more slender in males than in females. Thoracic sternum having median tooth or keel on each of fifth to eighth somite in males and non-spawning females, these structures reduced and sternum depressed or concave in spawning females. Consequently, species comparison using meristic characters between different sexes should be made with caution.

Composition. Forty-one species, including seven new species described in this study, are now known in the genus (Table 1).

Geographical and bathymetrical ranges. World-wide except for polar seas; sublittoral to 2850 m (Table 1).

Remarks. Members of Metacrangon are easily recognized by the usual presence of submedian teeth on the slightly depressed gastric region of the carapace and the possession of dorsolateral carinae on the sixth abdominal somite, although the submedian teeth are absent or rudimentary in M. clevai n. sp. and M. trigonorostris , or individually variable in the development in M. rau (cf. Komai & Ahyong 2011). Christoffersen (1988) indicated the monophyly of Metacrangon and considered the sister group to it, being a group containing Argis , Sclerocrangon and Rhynocrangon . The monophyly of the genus was supported by two apomorphic characters, viz., “Gastric region depressed below general level of carapace in adult” (character 10), and “Lateral surface of sixth abdominal somite in adult with a strong longitudinal carina” (character 65). However, the shallowly depressed gastric region of the carapace is also seen in Mesocrangon and Argis , and consequently this character is not autapomorphic for Metacrangon . Nevertheless, it has been confirmed that all species here assigned to Metacrangon have a distinct dorsolateral carina on the sixth abdominal somite, making the pleuron appear flared laterally. This character is not seen in other crangonid genera, and thus could be autapomorphic for Metacrangon .

Burukovsky (2003) proposed to transfer Metacrangon trigonorostris to the genus Argis based mainly on the absence of submedian teeth on the carapace and the similarities seen in the spatulate structure of the dactyli of the fourth and fifth pereopods. Komai & Komatsu (2009), however, did not agree with Burukovsky’s (2003) action because M. trigonorostris is clearly excluded from Argis by the structure of the frontal region of the carapace and the presence of distinct dorsolateral carinae on the sixth abdominal somite. Examination of the present material further supports Komai & Komatsu’s (2009) view. The submedian teeth on the carapace are reduced in M. trigonorostris , but there is sometimes a trace of low elevations on the corresponding position on the carapace, suggesting a possibility of the secondary loss of submedian teeth. The structure of the frontal region of the carapace is quite different between M. trigonorostris and species of Argis . In Argis , the orbit is formed by the frontal region of the carapace consisting of a greatly reduced rostrum, approximated antennal teeth and orbital lobes separated from antennal teeth by short, narrow notches; the eyestalks are contiguous and can be retracted in the specialized orbit ( Komai & Amaoka 1992; Komai 1994, 1997a). The structure of the frontal region of M. trigonorostris is quite similar to all other species of Metacrangon and allied crangonid genera, clearly excluding this species from Argis .

Yaldwyn (1960) proposed two informal species groups corresponding to members of Metacrangon , viz. the M. jacqueti group and M. munita group (as Sclerocrangon ). Komai (1997b) argued the possible monophyly of the M. jacqueti group, whereas the monophyly of the M. munita group is unclear because the identification of possible autapomorphic characters defining this group is difficult. The 21 western Pacific species treated in this study could be divided following the informal species group classification as follows:

Metacrangon jacqueti species group: M. bythos n. sp., M. ochotensis , M. punctata n. sp. and M. similis .

Metacrangon munita species group: M. asiaticus , M. clevai n. sp., M. cornuta , M. holthuisi , M. karubar , M. laevis , M. longirostris , M. miyakei , M. monodon , M. nipponensis , M. obliqua n. sp., M. proxima , M. robusta , M. sinensis , M. trigonorostris , M. tropis n. sp. and M. tsugaruensis n. sp.